An optical waveguide device using, for example, an electrooptical crystal substrate, such as an LiNbO3 (LN) substrate and LiTaO2 substrate, or a semiconductor substrate, such as GaAs substrate and InP substrate, is known as an optical module. This optical waveguide device is created by forming a metal film made of titanium (Ti) on part of the substrate and causing the film to thermally diffuse, or by executing proton exchange in a benzoic acid after patterning treatment. An electrode is then formed near the optical waveguide to construct the optical module, such as optical modulator.
When such an optical modulator is driven at high speed, the terminal of a signal electrode is assumed to be connected to the terminal of a ground electrode via resistance, thereby forming a traveling wave electrode, and a high-speed microwave signal (electronic signal) is applied from the input side to an RF terminal. At this time, an electric field shifts the refraction factors of a pair of parallel waveguides A and B by +Δ and −Δ, respectively, which consequently shifts the phase difference between the parallel waveguides A and B. In this manner, the signal passing through a Mach-Zehnder interferometer is output from an outgoing waveguide, as intensity-modulated signal light.
By matching the speed of light and the speed of the high-speed microwave signal (electronic signal), high-speed optical response characteristics can be achieved. After traveling through the optical modulator, the electronic signal travels through a capacitor and is terminated at a terminal resistor. Before the capacitor, the electrode is branched into one portion that extends through a bias resistor to be connected to a DC terminal and the other portion that is terminated at a terminal resistor. The optical modulator with this configuration functions as a bias. By applying a voltage to the DC terminal, a bias point and a drive voltage of the Mach-Zehnder unit can be controlled.
Such an optical modulator includes a Mach-Zehnder modulator unit and a relay substrate to which an electronic signal for driving the Mach-Zehnder modulator unit is input. As a technique related to the relay substrate, for example, a technique is known according to which the Mach-Zehnder modulator unit is disposed between a signal input substrate and a signal terminating substrate having a terminal resistor, etc. (see, e.g., Japanese Laid-Open Patent Publication Nos. 2007-139987 and 2003-015096). Another technique is also known according to which a signal input substrate and a signal terminating substrate are disposed on one side of a modulator (see, e.g., Japanese Laid-Open Patent Publication No. 2003-295139). Still another technique is also known according to which a relay substrate is provided as a unit separated from to an optical modulator unit and electrode intervals between RF terminals on the relay substrate are widened (see, e.g., Japanese Laid-Open Patent Publication No. 2010-185979).
In recent years, accompanying advances in techniques for multi-value processing and optical-polarization division multiplexing aimed at large-capacity optical communication, the configuration of the modulator has become more complicated. For example, a modulation method is adopted for the modulator such that two sets of Mach-Zehnder modulator units each having a pair of parallel waveguides are provided and an independent signal is input to the two sets of Mach-Zehnder modulator units to generate multi-value signals multiplexed by optical-polarization division.
According to the configuration that includes the two sets of Mach-Zehnder modulator units, however, the number of signal paths for electronic signals is doubled on a substrate having the Mach-Zehnder modulator units, which brings about a need for space to arrange the signal paths. Accordingly, the number of RF terminals, DC terminals, capacitors, bias resistors, and terminal resistors is also doubled on the relay substrate. This brings about a problem in that the size of the relay substrate, e.g., a length along the parallel waveguides of the Mach-Zehnder modulator units increases and the size of a module accommodating the Mach-Zehnder modulator units increases as well.